The present invention relates, in general, to a method for controlling the reception of data packets in a receiving unit of a mobile station such that power consumption of the mobile station can be conserved, and to such a mobile station.
Such methods for controlling reception of data packets in a receiving unit of a mobile station in a mobile radio system, and such mobile stations having control apparatus for controlling reception of data packets in a receiving unit in the mobile station are known from the prior art. During a call, that is to say when connected to a present base station, and during readiness to receive, that is to say when in the idle state, a mobile station in a mobile radio system has to regularly search predetermined frequencies for data packets from adjacent base stations and has to recognize their identity while it is connected to the present base station. As a rule, the identity is recognized by decoding the so-called BSIC (Base Station Identity Code) in the synchronization data packet. To do this, each base station regularly transmits synchronization data packets. In the GSM Standard, the base stations transmit a synchronization data packet with a duration of one time slot every ten or eleven time frames, with eight time slots forming one time frame.
In the GSM Standard, during a call, a mobile station can monitor an adjacent channel every 26 frames, for a period of somewhat more than one time frame. This one time frame is the so-called idle frame, in which the mobile station does not interchange data with the present base station. The idle frame is used to monitor adjacent channels and, if necessary, to find and to decode synchronization data packets from adjacent base stations. In the GSM Standard, the base stations transmit five synchronization data packets, with a duration of one time slot, within each multiple frame, which comprises 51 time frames. These synchronization data packets are transmitted by the base stations four times every ten frames, and then once after eleven frames. Since the mobile stations have an idle frame for monitoring the adjacent channels once in each 26 time frames, a mobile station can receive a synchronization data packet from an adjacent base station at the latest after eleven idle frames, irrespective of the relative timing of the synchronization data packets. Rather than monitoring a specific adjacent channel in two successive idle frames, another known option is to switch the mobile station to receive data packets from the adjacent base station in every other idle frame. This method is somewhat more elegant, but doubles the mean time that passes before reception of a synchronization data packet from an adjacent base station.
If a mobile station is ready to receive, that is to say in the idle state, more time is available for monitoring adjacent channels, since there is no need to interchange data continuously with the present base station. The mobile station can then continuously be switched to receive data packets from in each case one adjacent base station, and thus recognizes a synchronization data packet from an adjacent base station at the latest after eleven time frames. If it is not possible to receive data packets from one adjacent base station since, for example, the distance between the mobile station and the base station is too great or there is excessive interference on the transmission path, then this is confirmed after eleven time frames when the mobile station is ready to receive and after 26xc3x9711 time frames during a call.
In the method known from the prior art and the apparatus for controlling reception of data packets in a mobile station, the receiving unit of the mobile station is switched to receive synchronization data packets. That is to say a search is carried out for synchronization data packets from the respective adjacent base stations. The mobile station uses the synchronization data packets to synchronize itself to the adjacent base stations, in order to allow a handover from a present base station to an adjacent base station. In the GSM Standard, the base stations always transmit a frequency correction data packet one time frame before the synchronization data packets. This frequency correction data packet is used to match the frequency of the crystal oscillator in the mobile station to the base station when a link is set up (or set up again) from a mobile station to an adjacent base station. Since the crystal oscillator in the mobile stations is not generally sufficiently frequency-stable, the mobile station uses the frequency correction data packet to determine the frequency of the base station relative to the frequency of the crystal oscillator when it synchronizes itself to a base station for the first time.
A disadvantage of the methods and apparatus known from the prior art for controlling reception of data packets in a receiving unit of a mobile station in a mobile radio system is that the receiving unit in the mobile station is switched on in each time frame associated with a specific adjacent base station (for example the idle frames during a call), in order to attempt to receive synchronization data packets from the respective base station. This means that the receiving unit is switched to receive synchronization data packets even if there is no chance whatsoever of being able to receive synchronization data packets from the respective base station. However, switching on the receiving unit in the mobile station costs energy and reduces the call time and the time during which the mobile station is ready to receive.
It is accordingly an object of the present invention to provide a method and an apparatus to ensure that power consumption in the mobile station is reduced.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for controlling reception of data packets in a receiving unit of a mobile station in a mobile radio system. The receiving unit can in each case be switched during predetermined time frames to receive data packets to be detected from base stations, in which case a plurality of predetermined time frames are each assigned to receive data packets from a specific base station. The base station transmits characteristic data packets at a predetermined time interval before the data packets to be detected. The receiving unit is also switched to receive characteristic data packets from the respective base station during the predetermined time frames in which it is switched to receive data packets to be detected. If it does not receive either a data packet to be detected or a characteristic data packet from the respective base station in a predetermined time frame, the receiving unit is not switched to receive data packets from this base station during the next predetermined time frame which is assigned to this base station because no data packet to be detected from this base station can occur.
In accordance with the invention, a mobile station in a mobile radio system is correspondingly provided with a control apparatus for controlling reception of data packets in a receiving unit in the mobile station. The control apparatus can in each case switch the receiving unit during predetermined time frames to receive data packets to be detected from base stations. A plurality of predetermined time frames are in each case assigned to receive data packets from a specific base station, and the base stations transmit characteristic data packets at a predetermined time interval before the data packets to be detected. The control apparatus also switches the receiving unit to receive characteristic data packets from the respective base station during the predetermined time frames in which it switches this receiving unit to receive data packets to be detected. If it does not receive either a data packet to be detected or a characteristic data packet from the respective base station in a predetermined time frame, the receiving unit does not switch to receive data packets from this base station during the next predetermined time frame that is assigned to this base station because no data packet to be detected from this base station can occur.
In this first embodiment of the present invention, the receiving unit is thus switched to receive both data packets to be detected and characteristic data packets for adjacent channel monitoring. In the process, similar algorithms can be used for the recognition and evaluation of the data packets to be detected and the characteristic data packets in the mobile station, since both types of data packets are distinguished by a long, known bit sequence. The first embodiment of the present invention is based, in particular, on the fact that, if neither a characteristic data packet nor a data packet to be detected is received in a time frame, there cannot be a data packet to be detected in the next time frame, but at most one characteristic data packet. The reason for this is that the base stations always transmit a characteristic data packet one time frame before each data packet to be detected. If no characteristic data packet has been received by the receiving unit in the mobile station, it is thus not possible for a data packet to be detected to occur in the next time frame. There is thus no need to monitor this adjacent base station during this time frame.
In the ready-to-receive state and when the mobile station is switched on, this thus relates to the immediately following time frames since, in the ready-to-receive state, the mobile station uses a plurality of directly successive time frames for monitoring a specific base station. If the receiving unit does not receive either a data packet to be detected or a characteristic data packet in a predetermined time frame, then it can be certain that no data packet to be detected from this base station can occur in the immediately following time frame. Thus, the receiving unit can remain switched off during this immediately following time frame, thus saving a considerable amount of power in the mobile station.
During a call in the GSM System, the monitoring of a specific adjacent base station is generally carried out every 26 time frames, during the idle frame. Since the fundamental repetition rate of the sequence of a (characteristic) frequency correction data packet and of a synchronization data packet (to be detected) from the base stations is 51 time frames, the receiving unit in the mobile station can thus remain switched off in the next-but-one (one after the next) predetermined time frame if it does not receive either a synchronization data packet or a frequency data packet from the respective adjacent base station in a predetermined time frame. In the GSM System, this next-but-one time frame is the next-but-one idle time frame, which occurs again after (2xc3x9726=) 52 time frames. Since the repetition rate of the frequency correction data packets and data packets to be detected from the base station is 51 time frames, it is thus possible to predict confidently that, if neither a synchronization data packet nor a frequency correction data packet occurs in a specific time frame, no synchronization data packet from the same base station can occur after (51+1=) 52 time frames. The receiving unit in the mobile station can thus remain switched off during the next-but-one time frame once again this results in a considerable power savings.
When the mobile station is in the ready-to-receive state, the first embodiment of the present invention reduces, by virtually 50% (to be more precise: {fraction (7/16)}=44%) , the time during which the receiving unit must be switched on to search for adjacent channels. This results in a considerable power saving and thus in the time during which the mobile station is ready to receive being considerably extended.
In accordance with another feature of the invention, there is provided a second embodiment of the method for controlling reception of data packets in a receiving unit of a mobile station in a mobile radio system, in which the receiving unit can in each case be switched for predetermined time frames to receive data packets which are to be detected from base stations. A plurality of predetermined time frames are each assigned to receive data packets from a specific base station, and the base stations transmit characteristic data packets at a predetermined time interval before the data packets to be detected. The receiving unit is also switched to receive characteristic data packets and normal data packets from the respective base station during the predetermined time frames in which it is switched to receive data packets to be detected. The normal data packets each have a training sequence. The receiving unit in each case receives at least one normal data packet from the respective base station at the start of a time frame which is assigned to a specific base station. In order to determine the position of the training sequence in the normal data packets from the respective base station, the receiving unit is switched to receive data packets from the respective base station only during those times which correspond to the training sequences of the normal data packets in the time frames which are assigned to the respective base station.
In accordance with the second embodiment of the invention, there is provided a mobile station in a mobile radio system, having a control apparatus for controlling reception of data packets in a receiving unit in the mobile station. The control apparatus can in each case switch the receiving unit during predetermined time frames to receive data packets to be detected from base stations. A plurality of predetermined time frames are in each case assigned to receive data packets from a specific base station, and the base stations transmit characteristic data packets at a predetermined time interval before the data packets to be detected. The control apparatus also switches the receiving unit to receive characteristic data packets and normal data packets from the respective mobile station during the predetermined time frames in which it switches this receiving unit to receive data packets to be detected. The normal data packets each have a training sequence, and the receiving unit in each case receives at least one normal data packet from the respective base station at the start of a time frame which is assigned to a specific base station. Using the normal data packet, an evaluation unit determines the position of the training sequence in the normal data from the respective base station, whereupon the control apparatus, in the time frames which are assigned to the respective base station, switches the receiving unit to receive data packets from the respective base station only during the times which correspond to the training sequences of the normal data packets.
This second embodiment of the present invention is thus based on the fact that the receiving unit in the mobile station can receive characteristic data packets, data packets to be detected, and normal data packets. From receiving at least one normal data packet, the mobile station can determine the position of the training sequences in the normal data packets, at which point the receiving unit is switched to receive data packets from the adjacent base station only during the times which correspond to the position of the training sequences in the normal data packets. This prevents the receiving unit from being switched to receive data packets from the respective base station during those times in which the reception and the detection of characteristic data packets or data packets to be detected is impossible, or scarcely possible. This also results in considerable power savings in the mobile station.
The method and the mobile stations of the first and second embodiments of the present invention can advantageously be combined, by which means it is possible to achieve an even greater power saving in a mobile station.
Advantageously, with regard to the second embodiment of the present invention and with regard to the combination of the first and second embodiments of the present invention, the receiving unit is switched by the control apparatus to receive data packets from the respective base station in the time frames which are assigned to the respective base station only in a portion of the time periods which correspond to the training sequences in the normal data packets. The portion of the time periods is defined in such a manner that the data contained in this portion is sufficient to distinguish various types of data packets. Since the training sequences in the normal data packets and the bit sequences (which are located at an appropriate point) in the data packets to be detected and characteristic data packets are defined from the start and are different, two bits are sufficient in the worst case to recognize and distinguish the various types of data packets. In the worst case, it is thus sufficient to switch the receiving unit to receive data packets from the base station only during two bits. This allows a further major reduction in the power consumed in the mobile station.
With regard to the second embodiment of the present invention and the combination of the first and second embodiments of the present invention, the control apparatus advantageously determines the exact position of the training sequence in the normal data packets by comparing the training sequence in the at least one received normal data packet with possible training sequences that are stored in a memory in the mobile station. In the GSM Standard, for example, the normal data packets may have up to nine different bit sequences in the training frequencies. Recognition of a training sequence in a normal data packet is thus relatively complex. It is thus advantageous for the received bit sequences to be compared with possible training sequences in order to confirm whether a training sequence is actually present in a normal data packet, and where its exact position is. In this case, the accuracy is increased if the receiving unit can receive a plurality of normal data packets at the start of a specific time frame, since only the correct training sequences are, strictly, repeated in the time frame pattern of the normal data packets.
In both embodiments of the present invention, it is advantageous if the receiving unit receives a characteristic data packet from the respective base station in a predetermined time frame, and the receiving unit is not switched back to receive data packets from this base station until time between the data packets to be detected and the characteristic data packets. Since, in the GSM Standard, a (characteristic) frequency correction data packet is always transmitted by the base stations one time frame before a synchronization data packet (to be detected), it is thus possible, when a frequency correction data packet is received, for the receiving unit to remain switched off for one time frame, since the position of the synchronization data packet is in this case exactly known.
Furthermore, in both embodiments of the present invention, if it is not switched to receive data packets to be detected and characteristic data packets from the respective base station during one of the next predetermined time frames, the receiving unit is advantageously switched to receive data packets from another base station by the control apparatus. This is advantageous particularly when the mobile station is in the called state, since base stations can be found more quickly. Once again, this has the advantage of reducing the power consumption since, during a handover, it is possible to switch more quickly to a base station which may be transmitting more strongly. Furthermore, in fully extended mobile radio networks, this allows the cell size to be correspondingly reduced and, in consequence, the call capacity to be increased. Furthermore, mobile stations which are moving through small cell networks relatively quickly (cars in a town) lose a call if they cannot find new adjacent base stations sufficiently quickly. This problem is also solved by this refinement of the present invention.
Furthermore, in both embodiments of the present invention, it may be advantageous if the data packets received by the receiving unit in a predetermined time period from a respective base station are stored in a memory and are evaluated after the predetermined time period by the control apparatus. This is advantageous particularly if the signal processing in the mobile station is slow, in which case the received data packets cannot be identified in real time. In the GSM System, the data packets to be detected are synchronization data packets, and the characteristic data packets are frequency correction data packets.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and apparatus for controlling the reception of data packets in a mobile station, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.